数据库的事务

START TRANSACTION;

-- 检查库存
SELECT stock FROM products WHERE id = ?;

-- 如果库存大于0，继续执行以下语句
UPDATE shopping_cart SET quantity = quantity + 1 WHERE user_id = ? AND product_id = ?;

INSERT INTO orders (user_id, product_id, quantity, total_price) VALUES (?, ?, ?, ?);

UPDATE products SET stock = stock - 1 WHERE id = ?;

UPDATE users SET balance = balance - ? WHERE id = ?;

COMMIT;

import java.sql.*;

public class ShoppingCartService {
    private static final String URL = "jdbc:mysql://localhost:3306/shopping_platform";
    private static final String USER = "root";
    private static final String PASSWORD = "password";

    public void purchaseProduct(int userId, int productId, int productQuantity, double total_price) {
        Connection conn = null;
        PreparedStatement checkStockStmt = null;
        PreparedStatement updateCartStmt = null;
        PreparedStatement createOrderStmt = null;
        PreparedStatement updateStockStmt = null;
        PreparedStatement updateUserBalanceStmt = null;

        try {
            conn = DriverManager.getConnection(URL, USER, PASSWORD);
            conn.setAutoCommit(false); // 开始事务

            // 检查库存
            checkStockStmt = conn.prepareStatement("SELECT stock FROM products WHERE id = ?");
            checkStockStmt.setInt(1, productId);
            ResultSet rs = checkStockStmt.executeQuery();
            if (!rs.next() || rs.getInt("stock") < 1) {
                throw new Exception("Product out of stock.");
            }

            // 更新购物车
            updateCartStmt = conn.prepareStatement("UPDATE shopping_cart SET quantity = quantity + 1 WHERE user_id = ? AND product_id = ?");
            updateCartStmt.setInt(1, userId);
            updateCartStmt.setInt(2, productId);
            updateCartStmt.executeUpdate();

            // 创建订单
            createOrderStmt = conn.prepareStatement("INSERT INTO orders (user_id, product_id, quantity, total_price) VALUES (?, ?, ?, ?)");
            createOrderStmt.setInt(1, userId);
            createOrderStmt.setInt(2, productId);
            createOrderStmt.setInt(3, productQuantity);
            createOrderStmt.setDouble(4, total_price);
            createOrderStmt.executeUpdate();

            // 更新库存
            updateStockStmt = conn.prepareStatement("UPDATE products SET stock = stock - 1 WHERE id = ?");
            updateStockStmt.setInt(1, productId);
            updateStockStmt.executeUpdate();

            // 扣款操作
            updateUserBalanceStmt = conn.prepareStatement("UPDATE users SET balance = balance - ? WHERE id = ?");
            updateUserBalanceStmt.setDouble(1, total_price);
            updateUserBalanceStmt.setInt(2, userId);
            updateUserBalanceStmt.executeUpdate();

            conn.commit(); // 提交事务
        } catch (Exception e) {
            if (conn != null) {
                try {
                    conn.rollback(); // 回滚事务
                } catch (SQLException ex) {
                    ex.printStackTrace();
                }
            }
            e.printStackTrace();
        } finally {
            // 关闭资源
            try {
                if (checkStockStmt != null) checkStockStmt.close();
                if (updateCartStmt != null) updateCartStmt.close();
                if (createOrderStmt != null) createOrderStmt.close();
                if (updateStockStmt != null) updateStockStmt.close();
                if (updateUserBalanceStmt != null) updateUserBalanceStmt.close();
                if (conn != null) conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}